The invention relates to the measuring of the length of a signal propagation path. More particularly, the invention relates time domain reflectometry measurements of the length of a signal propagation path having a fixed impedance.
Time domain reflectometry (TDR) is a method of determining cable length and is analogous to determining distance in radar applications. TDR is based on the principal that a wave front traveling along a fixed impedance will be reflected by a gross impedance mis-match. The amount of the reflected energy with respect to the impedance mismatch is beyond the scope of this mention. Also, for a wave to travel, there must be a fixed impedance or only gradual changes in impedance along the path of propagation as well as minimal resistive losses. The impedance of a transmission path is dependent upon the capacitance and inductance per distance of the transmission line. The exact relationship is beyond the scope of this mention.
TDR can be measured by a high frequency counter which counts the amount of time that passes between sending a signal down a transmission line and receiving the reflection. To measure with a resolution of 1 foot down a cable where the propagation of the signal is 1.5 ns (nanoseconds) per foot requires a very high frequency counter (333 MHZ is a likely candidate frequency). To measure using this method over long distances, the counter itself must be very large. Because of the large size and speed of the counter, this can be a very power consuming solution, but is not uncommon.
TDR can also be measured by turning the transmission line into a tuned element in an oscillator. The longer the cable, the lower the frequency, much like a tuning fork. The problems with this method are numerous. The oscillator must be able to run at a very wide bandwidth, determined only by the transmission cable length, this could result in a very difficult to design oscillator. Also, to convert the rate of frequency to distance would require substantial processing power.
A circuit consistent with the present invention is used for determining an indication of a length of a conductor. The circuit includes a terminal for connection to a conductor under test. A pulse source circuit is coupled to the terminal for providing a signal to the conductor for use in determining an indication of the length of the conductor. A frequency generator circuit, coupled to the pulse source circuit, produces first and second frequency signals having a particular relationship. A mixer, coupled to the frequency generator circuit and the pulse source circuit, receives signals related to the first and second frequency signals and mixes the received signals to produce an output signal.